WASHINGTON -- Army Gen. David H. Petraeus yesterday told Congress that the deployment of 30,000 more troops to Iraq has made enough progress that the additional combat forces can be pulled out by next summer, but he cautioned against "rushing to failure" with a larger and speedier withdrawal.

In what some called the most anticipated congressional testimony by a general since the Vietnam War, Gen. Petraeus presented an upbeat picture of improving security conditions in Iraq and offered a grim forecast of the "devastating consequences" of a more rapid pullout. Gen. Petraeus said his forces "have dealt significant blows" to al-Qaida in Iraq, but warned that Iran is now fighting a "proxy war" against Iraqi and U.S. forces there.

The partial troop pullout that Gen. Petraeus outlined in a joint appearance with U.S. Ambassador to Iraq Ryan C. Crocker represents a modest acceleration of what military planners were privately forecasting, but it was the first drawdown they have publicly supported since becoming the top U.S. military and civilian officials in Baghdad.

President Bush, in a televised national address later this week, is likely to adopt the recommendation for this rollback of his troop "surge," while war critics immediately condemned it as too little, too late.

Either way, the general's report and troop proposal opened a new phase in the fractious Washington debate over the future of the U.S. venture in Iraq nearly 4 ¹/₂ years after Mr. Bush ordered an invasion to topple Saddam Hussein. From this point on, the argument will no longer be about whether to withdraw U.S. troops, but insteaad about how many to pull out and how quickly.

Gen. Petraeus' plan would rotate 2,200 Marines out of Anbar province in western Iraq this month without replacing them, then begin pulling out 17,500 Army troops and 2,000 more Marines starting in mid-December. If logistics personnel and other supporting troops were also withdrawn, that would return force levels to the "pre-surge" number of 130,000 by mid-July. The general asked Congress to defer decisions on further reductions until March to get a better sense of the political and security situation.

"Like Ambassador Crocker, I believe Iraq's problems will require a long-term effort," Gen. Petraeus testified before the House Armed Services and Foreign Affairs committees. "There are no easy answers or quick solutions. And though we both believe this effort can succeed, it will take time. ... A premature drawdown of our forces would likely have devastating consequences."

Some Democrats challenged Gen. Petraeus, although respectfully and with few sparks. Foreign Affairs Committee Chairman Tom Lantos, D-Calif., complained that the general depicted the options as a choice between a nominal drawdown and a precipitous pullout, without considering any middle ground. "Juxtaposing your token proposal with a hypothetical, rapid and irresponsible proposal does not do justice to this most important issue," Mr. Lantos said.

"What I recommended was a very substantial withdrawal," Gen. Petraeus countered.

Republicans, by contrast, seized on the plan as a political lifeboat after months of being forced to vote against measures repudiating Mr. Bush's policy. "Let the generals in the field dictate," said Rep. Patrick T. McHenry, R-N.C., an Iraq war veteran.

House Democratic leaders met yesterday in the office of Speaker Nancy Pelosi, D-Calif., to plot their next step. While House Democratic Caucus Chairman Rahm Emanuel of Illinois called Gen. Petraeus' drawdown "positive," Democrats said the House is likely to vote on three bills that would go further -- mandating a withdrawal timetable for all combat forces or specified rest time for units between deployments, and giving Mr. Bush 60 days to send Congress a more detailed pullout plan.

That would give Senate Democrats a menu to try to attract enough GOP support to break filibusters with 60 votes. Senate Majority Leader Harry M. Reid, D-Nev., and Minority Leader Mitch McConnell, R-Ky., are negotiating terms of another Iraq floor debate that could begin next week. Mr. McConnell said the goal is "to forge some sort of bipartisan consensus over a long-term strategy for Iraq and the Middle East."

Conservative Rep. John Tanner, D-Tenn., has persuaded a group of moderate Republicans to sit down with House Majority Leader Steny H. Hoyer, D-Md., to discuss options. Rep. Chris Van Hollen of Maryland, the Democratic Congressional Campaign Committee chairman, vowed to produce legislation that "would bring a meaningful change of direction in Iraq and get enough Republicans to get through the Senate."

Talk of collaboration reflects a shift from last spring, when Democrats appeared more interested in forcing party-line votes meant to put Republicans on the defensive. But liberal Democrats continue to push the caucus to be firmer on the war.

What was proffered last spring as a fall status report by Gen. Petraeus and Mr. Crocker became an anticipated moment of truth, with the update even codified into law, along with a requirement that the president assess whether Iraq had met 18 security, political and economic goals.

Gen. Petraeus showed up at the hearing room yesterday in his crisp uniform covered with medals, after weeks in which he has been the largely silent focus of a furious debate over his credibility and role. War opponents have assailed him as a shill for the White House, with the liberal group MoveOn.org even taking out a full-page advertisement in the New York Times dubbing him "General Betray Us" and accusing him of "cooking the books." Some analysts have debated the methodology of statistics used to report decreasing violence in Iraq.

A succession of protesters likewise made their antipathy for the general known at various points during the six-hour hearing yesterday. "How can you thank him for his service when we're slaughtering Iraqi civilians?" one woman shouted. An irked Armed Services Committee Chairman Ike Skelton, D-Mo., who ran the hearing, had disruptors removed and arrested.

One after the other, Republicans leapt to Gen. Petraeus' defense and called on Democrats to renounce MoveOn.org. The general addressed the dispute over whether he was pressured from the start, noting that although he had briefed Mr. Bush and his commanders about his assessments and recommendations, he wrote the testimony himself and did not show it to the Pentagon or White House beforehand. He also defended the "rigor and consistency" of his command's statistics.

The numbers he presented pointed to improvements. Security incidents have declined in eight of the past 12 weeks, and in the past two weeks reached the lowest levels since June 2006, he said. Civilian deaths, he added, are down 45 percent since December and 70 percent in Baghdad, while sectarian deaths have fallen by 55 percent. And he said U.S. forces have killed or captured 100 leaders of al-Qaida in Iraq and 2,500 rank-and-file fighters.

It fell to Mr. Crocker to acknowledge that Iraqi political leaders have not made much progress in forging national accord, the original goal of the surge, although he insisted that "the seeds of reconciliation are being planted."

Mr. Crocker, a longtime diplomat in the Middle East, appealed for understanding, tracing the Iraqis' problems in reaching consensus to the brutality of Saddam's government and comparing the issues confronting them with those of slavery and civil rights that tore apart the United States for so long.